A positioning device may be used for positioning a part of a complex structure such as a chair. The positioning device is constructed as a fluid-operated spring unit which biases the movable part in a predetermined direction. The fluid-operated spring can be adjusted into a plurality of operative lengths. The operative length is responsible for the respective position of the movable part. The adjustment of the movable part is performed in one direction by acting on the movable part against the action of the spring unit. After the desired position has been achieved, the spring unit is locked in usual way. In some cases, it is desirable to make the resistance of the spring unit variable in dependence of another force which occurs within the construction. One possible case is a chair. In such a chair the spring unit may be used for adjusting the back-rest in a position adapted to the anatomy of the user. For adjustment of the back-rest the user urges by his back onto the back-rest against the action of the spring unit after the spring unit has been unlocked. It is desirable that the spring resistance of the back-rest, which depends on the spring force of the spring unit, is adapted to the weight of the user such that the user can move by his back said back-rest without difficulty, on the one hand, and cannot tilt suddenly backwards due to insufficient spring resistance of the spring unit, on the other hand.